The invention relates to a display device having a display panel comprising a first substrate provided with a first picture electrode and a second substrate provided with a second picture electrode, both electrodes overlapping at least partly and both substrates sandwiching an electro-optical medium at least at the location of the overlap between both picture electrodes, said electro-optical medium being capable of switching between an at least substantially transparent state and a scattering state under the influence of an electric field, and said display device comprising a light source which sidelong illuminates the display panel at least during operation.
The electro-optical medium is generally formed by a layer comprising a liquid-crystalline material, in which case such a device is commonly referred to as LCD which is the abbreviation for Liquid Crystal Display.
The invention particularly, although not exclusively, relates to a display device in which the electro-optical medium is formed by a combined system of a birefringent liquid-crystalline material and a suitable polymer, and said liquid-crystalline material may be dispersed in a polymer matrix, for example, in the form of droplets. An electro-optical medium of this type is generally referred to as PDLC, which is the abbreviation for Polymer Dispersed Liquid Crystal.
In the absence of an electric field, the birefringent droplets in such a medium are randomly oriented and incident light will be randomly scattered at the interface between the droplets and the polymer matrix and between the droplets themselves. Under the influence of an electric field, however, the droplets are oriented in conformity with the electrical lines of force, so that incident light has a uniform refractive index in the liquid-crystalline material. By a suitable choice of the liquid-crystalline material on the one hand and the polymer material on the other hand, it can be achieved that this uniform refractive index substantially corresponds to the refractive index of the polymer matrix, so that scattering occurs neither at the interface between the polymer matrix and the droplets nor between neighbouring droplets, as a result of which the system is transparent to incident light. Thus, the electro-optical medium is capable of switching between a scattering state and a substantially transparent state under the influence of an electric field.
A device of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,099,343. In said known device the display panel comprises two substrates in the form of flat plates of glass or a suitable transparent synthetic resin, siad plates sandwiching, over substantially their entire surface, an electro-optical medium in the form of a layer of PDLC material.
In the known device, a light source in the form of an elongated fluorescent lamp is arranged at the side of the display panel and illuminates the whole side of the panel. The emitted light is captured by the display panel and can propagate through the panel by means of internal reflection. The light is reflected at the outer surfaces, i.e. the sides facing away from the PDLC layer, of both transparent plates.
The inner surface of both plates, which are in contact with the PDLC material, are provided with a picture electrode and the electrodes overlap. The overlap between the two picture electrodes determines the location of a picture element where the liquid-crystalline material can switch between both states under the influence of an electric field which is applied between said two picture electrodes. In the scattering state, light will be emitted at the location of the picture element, so that an observer sees the picture element luminesce. However, if the liquid-crystalline material is transparent at the location of the picture element, the light remains confined in the device and, hence, no light is emitted at the location of the picture element. In this case, an observer sees through the picture element and sees the (dark) background of the display panel.
In the known device, both picture electrodes must be substantially transparent to ensure that the light captured is not hindered. For this reason, indium-tin oxide (ITO), which is at least substantially transparent to visible light, is used for both picture electrodes.
Indium-tin oxide is electrically conductive, but its resistivity is much higher than that of most metals. For example, the sheet resistance of an average conductor track of aluminium typically is less than 0.5 .OMEGA./.quadrature., which is approximately a factor of 40 smaller than the sheet resistance of approximately 20 .OMEGA./.quadrature. of an average track of indium-tin oxide. Thus, particularly in the case of relatively long and narrow picture electrodes, ITO leads to a relatively high series resistance and hence to a relatively long RC time, which is an important disadvantage for, in particular, display devices having a relatively large number of picture elements. In addition, in comparison with, for example, many metals, indium-tin oxide is difficult to handle from a technological point of view.